1. Field of the Invention
The present invention relates to an absolute encoder that detects an absolute position of an object.
2. Description of the Related Art
Previously, as an apparatus that measures a moving distance of an object, an absolute encoder capable of performing a length measurement of an absolute position in addition to an incremental encoder that measures a relative moving distance has been known.
Japanese Patent Laid-Open No. 2006-170788 discloses an absolute encoder having one track in which a length of an incremental pattern changes in accordance with a position in a scale portion, which is configured so that a reflected light intensity changes in accordance with a relative moving amount of the scale portion and a sensor unit. This absolute encoder is configured to be able to detect an absolute position of an object by the change of the light intensity in accordance with the relative moving amount of the scale portion and the sensor unit.
Japanese Patent Laid-Open No. 2009-002702 discloses an absolute encoder having a three-track configuration by providing a scale portion with an exclusive track for obtaining absolute information.
However, in the absolute encoder disclosed in Japanese Patent Laid-Open No. 2006-170788, an error is accumulated when a variation of the light intensity of the light source is generated or an external noise is generated.
On the other hand, when the three-track configuration such as the absolute encoder disclosed in Japanese Patent Laid-Open No. 2009-002702, the size of whole of the absolute encoder including the sensor unit and the scale portion is enlarged, and the cost is increased. Furthermore, in the configuration disclosed in Japanese Patent Laid-Open No. 2009-002702, good detection accuracy in a vernier detection cannot be obtained because a signal cannot be obtained in an area where the signal amplitude is zero. In the method of the vernier detection, it is difficult to guarantee the synchronization of the absolute position in all strokes when a longitudinal direction of the scale portion is long in the scale portion having a two-track configuration. In order to avoid this accuracy deterioration, the size of the sensor unit needs to be enlarged in a moving direction of the scale portion, and it prevents the reductions of the size and the cost. Furthermore, it is difficult to uniformly illuminate a light in a wide range of the scale portion since the sizes of the scale portion and the sensor unit are enlarged, and it contributes to the accuracy deterioration.